The Carbon Farming Initiative and Agricultural Emissions

1. This presentation was prepared by the University of Melbourne for the Regional Landcare Facilitator training funded through the Australian Government’s Carbon Farming Initiative Communications Program The Carbon Farming Initiative and Agricultural Emissions

2. PART 3: THE NITROGEN CYCLE IN AGRICULTURAL SYSTEMS This presentation explains the nitrogen cycle in agricultural production systems and the various pools of nitrogen

3. The nitrogen cycle • Content • The nitrogen cycle in agricultural systems • Fluxes between nitrogen pools • Forms of nitrogen and their fate

4. The nitrogen cycle Whitehead 1995

5. The nitrogen cycle • Atmosphere • N2 (nitrogen) • 78% of the atmosphere • O2 = 21%, CO2 = 0.04% • Fixed by legumes into plants and soil • N2O (nitrous oxide) • 0.32 ppm (0.00003%) Eckard 2011

6. The nitrogen cycle • Main forms of soil & plant N • NH3 – Ammonia • Organic matter • Fertilisers • Urea, DAP, UAN etc. • Major source of plant N oxygen amide ion amide ion Eckard 2011

7. The nitrogen cycle • Main forms of soil & plant N • NH4+ – Ammonium • Soil solution • Loosely bound on cation exchange • Positive charge attached to clay • Exchangeable • Clay-fixed (non-exchangeable) • Does not readily leach • Major source of plant N (nitrogen) • Preferential uptake in colder, wetter soils • Rapidly converts to NO3- (nitrate ion) • In warm, well-drained soils Eckard 2011

8. The nitrogen cycle • Main forms of soil & plant N • NO3 (nitrate ion) • Major source of plant nutrition • Drier soils • Accumulates in some plants • e.g. Brassicas, annual ryegrass, kikuyu, cereal grains • Breaks down to NO2 in rumen – toxicity • Soluble in water – leaches • NO2 (nitrite ion) • Transient in plants and soils • Main form of toxicity in ruminants Eckard 2011

9. The nitrogen cycle • Main forms of soil & plant N • Soil organic matter N • Decomposed residues • Amides, proteins etc • Microorganisms (microbial biomass) • C: N ratio • Usually 10:1 to 40:1 • Major source of plant N • Through mineralisation Eckard 2011

10. NO3- N2O N2 The nitrogen cycle • Mineralisation • Microbial breakdown of soil organic matter to ammonium • The main mechanism for supplying N to plants • Nitrification • Microbial conversion of ammonium to nitrate • Ammonia sources • Urine, decaying organic matter, fertiliser • Warm, moist (not waterlogged) soils • Denitrification • Microbial conversion of nitrate to N2 and N2O gasses • Warm, waterlogged soils • N2O is a powerful greenhouse gas • Immobilisation • Microbial assimilation of soil nitrogen into OM OM organic matter NH4+ ammonium ion nitrate ion nitrogen nitrous oxide Eckard 2011

11. The nitrogen cycle • Nitrogen balances in Agricultural systems • Biological efficiency • Less than 50% Eckard et al 2007